Editing Serpentinization (section)

==Environment of formation==
[[Image:Gros Morne moho.jpg|thumb|Ophiolite of the [[Gros Morne National Park]], Newfoundland. Ophiolites characteristically have a serpentinite component.]]
Serpentinization occurs at [[mid-ocean ridges]], in the [[forearc]] mantle of [[subduction zone|subduction]] zones, in ophiolite packages,  and in ultramafic intrusions. <ref name="GeologyDictionary" /><ref name=":2" />

=== Mid-ocean ridges ===
Conditions are highly favorable for serpentinization at slow to ultraslow spreading mid-ocean ridges.<ref name=Mevel2003/> Here the rate of [[crustal extension]] is high compared with the volume of magmatism, bringing ultramafic mantle rock very close to the surface where fracturing allows seawater to infiltrate the rock.<ref name=Lowell2002/>

Serpentinization at slow spreading mid-ocean ridges can cause the seismic Moho discontinuity to be placed at the serpentinization front, rather than the base of the crust as defined by normal petrological criteria.<ref>{{cite journal |last1=Minshull |first1=T. A. |last2=Muller |first2=M. R. |last3=Robinson |first3=C. J. |last4=White |first4=R. S. |last5=Bickle |first5=M. J. |title=Is the oceanic Moho a serpentinization front? |journal=Geological Society, London, Special Publications |date=1998 |volume=148 |issue=1 |pages=71–80 |doi=10.1144/GSL.SP.1998.148.01.05|bibcode=1998GSLSP.148...71M |s2cid=128410328 }}</ref><ref name="Mevel2003" /> The Lanzo Massif of the Italian Alps shows a sharp serpentinization front that may be a relict seismic Moho.<ref>{{cite journal |last1=Debret |first1=B. |last2=Nicollet |first2=C. |last3=Andreani |first3=M. |last4=Schwartz |first4=S. |last5=Godard |first5=M. |title=Three steps of serpentinization in an eclogitized oceanic serpentinization front (Lanzo Massif - Western Alps): ECLOGITIZED SERPENTINIZATION FRONT (LANZO) |journal=Journal of Metamorphic Geology |date=February 2013 |volume=31 |issue=2 |pages=165–186 |doi=10.1111/jmg.12008|s2cid=140540631 }}</ref>

=== Subduction Zones ===

==== Forearc mantle ====
Serpentinization is an important phenomenon in subduction zones that has a strong control on the water cycle and geodynamics of a subduction zone. <ref name=":3">{{Cite journal |last1=Xia |first1=Shaohong |last2=Sun |first2=Jinlong |last3=Huang |first3=Haibo |date=2017-05-31 |title=Degree of serpentinization in the forearc mantle wedge of Kyushu subduction zone |url=http://dx.doi.org/10.1190/igc2017-238 |journal=International Geophysical Conference, Qingdao, China, 17-20 April 2017 |pages=941–943 |publisher=Society of Exploration Geophysicists and Chinese Petroleum Society |doi=10.1190/igc2017-238|url-access=subscription }}</ref> Here mantle rock is cooled by the subducting slab to temperatures at which serpentinite is stable, and fluids are released from the subducting slab in great quantities into the ultramafic mantle rock.<ref name=":3" /> Direct evidence that serpentinization is taking place in the [[Mariana Islands]] [[island arc]] is provided by the activity of serpentinite [[mud volcano]]es. [[Xenolith]]s of harzburgite and (less commonly) dunite are occasionally erupted by the mud volcanoes, giving clues to the nature of the protolith.<ref name=HyndmanPeacock2003>{{cite journal |last1=Hyndman |first1=Roy D |last2=Peacock |first2=Simon M |title=Serpentinization of the forearc mantle |journal=Earth and Planetary Science Letters |date=July 2003 |volume=212 |issue=3–4 |pages=417–432 |doi=10.1016/S0012-821X(03)00263-2|bibcode=2003E&PSL.212..417H }}</ref>

Because serpentinization lowers the density of the original rock, serpentinization may lead to uplift or exhumation of serpentinites to the surface, as has taken place with the serpentinite exposed at the [[Presidio of San Francisco]] following cessation of subduction.<ref name="Presidio">{{cite web |title=Serpentinite |url=https://www.nps.gov/prsf/learn/nature/serpentinite.htm |website=Presidio of San Francisco |publisher=National Park Service |access-date=3 September 2021}}</ref>
 
Serpentinized ultramafic rock is found in many [[ophiolite]]s. Ophiolites are fragments of oceanic [[lithosphere]] that has been thrust onto continents, a process called ''[[obduction]]''. <ref>{{Cite web |date=2010-04-15 |title=Ophiolites |url=https://volcano.oregonstate.edu/volcanic-minerals/ophiolites |access-date=2022-11-20 |website=Volcano World |language=en}}</ref> They typically consist of a layer of serpentinized harzburgite (sometimes called ''alpine peridotite'' in older writings), a layer of hydrothermally altered [[diabase]]s and [[pillow basalt]]s, and a layer of deep water sediments containing radiolarian ribbon [[chert]].<ref>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |pages=370–372}}</ref> 
[[File:Serpentinization Process at Subduction Zone.jpg|thumb|Hydration of [[forearc]] mantle due to the water expelled from deeper part of the subducting plate. Adapted from Hyndman and Peacock (2003)]]

=== Implications ===

==== Limitation on earthquake depth ====
[[Seismic wave]] studies can detect the presence of large bodies of serpentinite in the crust and upper mantle, since serpentinization has a huge impact on [[shear wave]] velocity. A higher degree of serpentinization will lead to lower shear wave velocity and higher [[Poisson's ratio]].<ref name=":1" /> Seismic measurements confirm that serpentinization is pervasive in forearc mantle.<ref name="HyndmanPeacock200322">{{cite journal |last1=Hyndman |first1=Roy D |last2=Peacock |first2=Simon M |date=July 2003 |title=Serpentinization of the forearc mantle |journal=Earth and Planetary Science Letters |volume=212 |issue=3–4 |pages=417–432 |bibcode=2003E&PSL.212..417H |doi=10.1016/S0012-821X(03)00263-2}}</ref> The serpentinization can produce an inverted [[Moho discontinuity]], in which seismic velocity abruptly ''decreases'' across the crust-mantle boundary, which is the opposite of the usual behavior. The serpentinite is highly deformable, creating an aseismic zone in the forearc, at which serpentinites slide at stable plate velocity. The presence of serpentinite may limit the maximum depth of [[Megathrust earthquake|megathrust earthquakes]] as they impede rupture into the forearc mantle.<ref name=":1">{{cite journal |last1=Bostock |first1=M. G. |last2=Hyndman |first2=R. D. |last3=Rondenay |first3=S. |last4=Peacock |first4=S. M. |date=May 2002 |title=An inverted continental Moho and serpentinization of the forearc mantle |journal=Nature |volume=417 |issue=6888 |pages=536–538 |bibcode=2002Natur.417..536B |doi=10.1038/417536a |pmid=12037564 |s2cid=3113794}}</ref>